


Across The Universes

by Mellowphant



Series: An Experiment [2]
Category: Undertale (Video Game)
Genre: Alternate Universe - Underswap (Undertale), Established Alphys/Undyne (Undertale), Eventual Romance, F/M, Fluff, Hurt/Comfort, Hypothermia, Reader Is Not Frisk (Undertale), Underswap Papyrus (Undertale), Underswap Sans (Undertale), Whump, reader is female
Language: English
Status: In-Progress
Published: 2019-03-28
Updated: 2019-04-03
Packaged: 2019-12-25 14:17:09
Rating: Not Rated
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 2
Words: 3,355
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/18263027
Author URL: https://archiveofourown.org/users/Mellowphant/pseuds/Mellowphant
Summary: It was dark. Then it wasn’t. You woke up in a strange place and no one knew how you got there, least of all you. Despite not having any memories, one thing is clear to you: You need to get home. You don’t know where “home” is or what getting there entails, but you’ll do what you have to. With the help of the two skeletons and their friends, you set out to learn the truth about who you are and how you ended up Underground.





	1. Hypothermia

**Author's Note:**

> Hey! This is an idea I’ve had for a while and I wanted to write some Underswap so here we are! I think this will be short and sweet (less than 20 chapters) and I might not update ultra regularly, but I wanted to put it out there because I like it and think it’s cool!

It’s dark. So dark. Has it always been dark? You don’t know. You can’t say how long you’ve been here, or how you arrived. Vague thoughts float through your mind, but they slip away like sand through your fingertips; questions of time and place that have no answers, a sense of purpose that disappears as soon as you notice it. You don’t know how long you’ve been where you are, but something tells you that it doesn’t matter. The darkness is all you know.

Then, a feeling.

You aren’t sure that’s what it is at first—it’s been so long song since you’ve felt, hasn’t it? But no, this is a feeling. You’re certain of it now. It prickles against your skin and you feel your body shudder in response. It’s familiar. It isn’t good, or bad; it’s merely a sensation. All at once, you feel something rocket through your body and your lips form the word.

 

 _Cold_.

 

This is cold. You feel cold. It’s as if your brain wakes up with the realization. You feel the electricity surge through your veins. There’s something pounding in your chest. The tips of your fingers brush against something soft. Something wet.

Something _cold_.

It kisses your face in tiny drops and you feel your eyelids flutter.

_It’s time to wake up._

You open your eyes. It’s bright. You have to squint. There’s a face in front of you, smiling with a big toothy grin.

“HELLO! WHO ARE YOU?”

Your teeth chatter as the sensation pricks your fingers and stings your cheeks.  You answer with the only thing you know. 

“C-cold.” 

“HELLO, COLD! I AM THE MAGNIFICANT SANS! THOUGH YOU MAY CALL ME JUST SANS IF YOU’D LIKE.”

 _Sans_. His eyes are bright blue stars, twinkling within two dark sockets. His face is round and smooth, his grin wide and bright. He almost looks like a skeleton, you think. Something about that feels strange, but you can’t understand why.

White flurries dance through the air. _Snow_ , your brain supplies. It falls in large flakes, clinging to Sans’s blue bandana. He stares at you, mouth slightly agape. There’s a soft blue glow emanating from his cheekbones. It looks warm.

“I-I MEAN JUST THE _NAME_ SANS. DO NOT CALL ME ‘JUST SANS.’ T-THAT IS OF COUSE UNLESS Y-YOU’D _LIKE_ TO CALL ME ‘JUST SANS’ IN WHICH CASE I DON’T MIND! I DON’T TYPICALLY ENJOY NICKNAMES BUT I SUPPOSE I COULD MAKE AN EXCEPTION IN THIS PARTICULAR CASE!”

The snowy world around you is spinning. You dig your fingers into the ground. They feel like they’re burning, yet they don’t feel anything at all. You’re shaking. It’s so cold. You’re on fire. 

“BUT WE CAN TALK ABOUT IT LATER. RIGHT NOW, YOU AND I HAVE SOME IMPORTANT BUSINESS TO ATTEND TO! YOU SEE, I AM PATROLING FOR….” 

His voice is loud. Too loud. Your head is pounding with a dull ache, your eyes are heavy with sleep. It wouldn’t be so bad to close them, would it? (Yes.) You could sleep it off, couldn’t you? (Don’t.) You won’t feel bad when you’re sleeping. You won’t feel anything at all. 

“H-HEY! COLD! WAKE UP!”

You feel something shake your shoulder. You open your eyes ever so slightly.

“I, THE MAGNIFICENT SANS, HAVE SUCCESSFULLY CAPTURED YOU! THAT MEANS I WILL NOW BE MAKING YOU MY PRISONER!” 

You try to form a response, but your lips refuse to cooperate. It comes out slurred and confused. You’re not sure what you were trying to say. They might not have even been words at all.

“H-HUH? UH, I DON’T THINK…”

You don’t have the energy to answer.

“UH! I-IT’S FINE! EVERYTHING WILL BE FINE! F-FEAR NOT!”

Suddenly, you feel two arms reach underneath you, scooping you up off the ground.

 _Warm_.

You instinctively pull yourself closer. The arms momentarily go stiff underneath you, but slowly relax again. They hold you tightly. You feel wind lash against your exposed skin and you turn your head away, burying it into something soft.

You close your eyes and prepare for the numbing darkness. But it doesn’t come. It’s a blur of color and light. There are sounds. Muffled voices, closing doors, footsteps on wood floors. You’re swaddled in warmth. You feel something soft against your cheek. It’s hazy and confusing, but you feel. The warmth beckons to you; coaxing you to sleep. 

 _No…p…please._  

Do you think it or do you say it? You don’t know.

Something shushes you.

“it’s ok. you can sleep.” 

_N….no…_

If you sleep, you might have to go back. Back to the darkness. Here isn’t dark. Here, the world is so full. You can’t go back there. There was nothing there. There was so much nothing.

“i’ll be here.”

_It was so dark…_

“i’ll leave the light on.”

_Promise?_

There’s a whisper, but you don’t catch it. You fall into a deep sleep. 

* * *

 Papyrus walks into the kitchen, softly closing the door behind him. As soon as his brother sees him, he stops his nervous pacing and rushes over.

“W-WILL IT BE OK?”

“I dunno,” he says with a shrug, instinctively reaching for a cigarette. He catches himself though and moves to rub the back of his neck instead. It’s an awkward gesture, but Sans thankfully doesn’t notice.

“HAS IT FALLEN DOWN? _W-WILL IT TURN TO DUST?!”_  

“i uh… i don’t think humans do that…”

Sans breathes a sigh of relief. “OH OK.”

The two stand in silence for a moment. He tries to act calm—for Sans’s sake more than anything—but his leg still bounces nervously. Should he check on you? He just saw you. He couldn’t do anything then, so there probably isn’t anything he can do now. Still, he shoots a quick glance in the direction of the living room.

He has so many questions.

When Sans kicked down the front door, claiming that he caught a human, Papyrus thought it was another one of his drills. No matter how many times Sans insisted that it could be an actual human, he laughed it off with a shrug and a smile. 

He didn’t think that day would be today, or quite frankly, ever. That changed when he saw his brother walk in with you in his arms. He didn’t even believe it until he heard Sans’s panicked voice asking for help. Even now, it hardly seems real. You don’t seem real.

_*where did you even come from?_

The guy in the ruins didn’t say anything about a human passing through. To get into the forest, you would’ve had to go past his sentry station. Sure, he hasn’t always been the most attentive when it comes to his job, but a human in the forest is something he would’ve noticed… right? 

“WAIT NO!”

The sound of Sans’s voice pulls him from his thoughts.

“THAT CAN’T BE RIGHT! HUMANS _DO_ FALL DOWN! THERE ARE DEATHS IN THE HISTORICAL HUMAN FILMS CAPTAIN ALPHYS AND I WATCH! I MEAN SOMETIMES THE HUMAN CHARACTER COMES BACK AND IT’S _VERY_ CONFUSING BECAUSE YOU THOUGHT THEY COULDN’T COME BACK BUT THEN THEY DIE AGAIN AND THEN THEY COME BACK AGAIN AND ALPHYS SAYS THAT IT’S A CHEAP TRICK TO CREATE DRAMA BUT SHE WATCHES THEM ANYWAY WHICH I DON’T ENTIRELY UNDERSTAND BECAUSE IF IT’S SO BAD THEN WHY DOES SHE-”

Papyrus nods, but he can’t say he’s really paying much attention. He walks over to the kitchen door, peeking through the crack. He can’t really see you from here, but the pile of covers you’re under hasn’t moved since the last time he looked so it’s safe to say you haven’t either. He sighs, turning back just in time to catch the tail end of his brother’s rambling.

“BUT THEN AGAIN THAT’S ONLY SOMETIMES SO I’M NOT SURE WHAT TO REALLY THINK ABOUT IT!”

“huh yeah…”

“PAPY!” he huffs. “YOU WEREN’T EVEN LISTENING!" 

“i was listening.”

“THEN WHAT DID I SAY?”

“alphys has bad taste in anime.”

“N-NO!” Sans shoots him a look of disbelief before crossing his arms with a huff. “WHAT I _SAID_ WAS THAT MAYBE WE CAN DO WHAT THEY DID IN SERPENT SQUARE X THE MOVIE! THEY BROUGHT ALL KINDS OF HUMANS BACK FOR THAT ONE!”

Somehow, _Serpent Square X_ doesn’t seem like a very reliable source when it comes to understanding humans. Then again, what does he know? What does _anyone_ know about humans? He’s only seen a human one other time and even then, he was very young. At this point, he’s not even sure if it even technically happened or if the entire thing was erased from history. The only person alive who’s truly met a human is Queen Toriel herself and her reputation with them hasn’t been particularly friendly. He once heard someone at Muffet’s say that the Queen had adopted a human child and raised it along with the lost prince. Still, the legends say that didn’t end very well and even those are only legends. Anyone who would’ve been around at the time of the fabled child has long since turned to dust. The truth is, they don’t know anything. Captain Alphys hasn’t even seen the thing she’s been trained to capture.

“…PAPY?”

“yeah?”

“WHAT IF IT FALLS DOWN AND IT’S MY FAULT?”

_*shit._

“h-hey c’mon sans! don’t say that!”

Sans doesn’t seem to notice his stuttered attempt at comfort, pacing around the kitchen even more intently than before. He nervously wrings his hands together, his chest rising and falling with panicked breaths.

_*shitshitshit_

“b-bro, i’m sure it’s fi-”

“IT SAID IT WAS COLD A-AND I D-DIDN’T EVEN-! I D-DIDN’T _KNOW_ HUMANS COULD FALL DOWN FROM THE C-COLD! A-AND IT’S SO _FRAGILE!_ I DIDN’T K-KNOW HUMANS W-WERE S-SO…WE DON’T EVEN KNOW ITS _NAME!_ ” Sans’s voice quavers as large tears form at the corners of his sockets. “W-WHAT IF IT HAS A FAMILY WHO’S LOOKING FOR IT AND-”

There’s the sound of faint coughing coming from the living room.

In a flash, they’re both standing at the door, trying to get a glimpse of you through the crack. They stay incredibly still as if moving will somehow make you sicker.

There’s another cough.

Sans rushes over before Papyrus can even voice a warning to stop him. He kneels down in front of the couch, looking at you with wide eye lights. It’s too close for Papyrus’s comfort. He feels his magic surge through his veins, just in case.

“Human? Are You Ok?”

The sound of his brother’s whisper nearly stops his soul. The last time Sans spoke so quietly was when-

The blankets rustle.

He reflexively jumps back, but Sans stays completely still, a single gloved hand raised tentatively towards the couch.

“Human?" 

“…ss….ans…sss…”

It’s barely above a whisper, but it’s unmistakable.

“I’m Here, Human! What Is It? What Do You Need?”

There’s a soft hum followed by an unintelligible mumble. Then silence. 

After another moment, Papyrus walks over, placing a hand on his brother’s shoulder.

“we should let the human rest.”

Sans nods but makes no motion to stand. He watches you carefully, his face hard with determination.

Papyrus has seen that expression before, and he knows there’s nothing he can do once he sees it. Sans’s seemingly endless energy is mostly harmless, but once he decides to be stubborn, that’s the end. He doesn’t throw tantrums, he just plants himself firmly in the ground and devotes his entire soul to staying put until he gets what he wants. Papyrus has never been able to outlast him; he just doesn’t have the same drive. Hardly anyone does. If he’s being honest, he’s surprised Alphys lasted as long as she did. Sans stood outside of her house for 4 days straight before she agreed to train him.

After a while of watching his brother, he sighs, a smile forming on his face. Sans only met you today, but he’s already willing to do what it takes to make sure you get better. His brother is the coolest.

_~~just hope it doesn’t get him killed.~~ _

He shakes it off, standing up and stretching his arms. It’s been a long day.

“hey, i was gonna make some hot chocolate. you want some?” He watches his brother carefully as he says it.

For a split second, Sans almost breaks concentration. He seems to catch himself, as he straightens back up, puffing out his cheeks. But Papyrus knows better. He leaves the question to hang in the air for just a little longer. Then, as if on cue, Sans slowly nods his head. Papyrus feels his shoulders relax slightly as he releases a silent sigh of relief. Things would’ve had to be really bad for Sans to say no to that. He gives his brother a pat on the back before standing up to go to the kitchen. The sound of Sans’s voice stops him in the doorway. 

“PAPY?” 

“hm?”

“DO YOU THINK THE HUMAN MIGHT WANT ONE WHEN IT WAKES UP?” 

He feels his body go stiff, if only for a second. He casually shoves his hands into his pockets and shrugs. Sans doesn’t seem like he noticed.

“how ‘bout i make it one, just in case?” 

Sans considers this for a moment before giving a decisive nod of his head. He then goes back to watching you with that intense focus of his. 

As soon as Papyrus makes it into the kitchen, he fumbles around his pockets for his cigarettes. He can’t light it without Sans noticing, but just holding one in his hand is enough for right now. He releases a shaky breath, leaning against the kitchen counter. Part of him wishes you wouldn’t wake up. It would be a lot easier for both of them if you didn’t. But Sans… _Stars_. He’d be heartbroken. His soul is too big for his own good. Out of everyone in the Underground, _he_ had to be the one to find you. It's just perfect. He curses softly to himself, shooting another quick glance into the living room. It’s exactly as he left it. He’s being paranoid. He drums his phalanges on the linoleum countertop, allowing the rhythmic taps to calm his nerves. After a few moments, he stops, looking in on the two of you one last time.

If there's one thing he can promise, it's that he's gonna get some answers. No matter what it takes.


	2. Reset

**Summary for the Chapter:**

> Somewhere in a universe far away...

**Notes for the Chapter:**

> I don't have this marked as a related work because it isn't really, but this chapter was centered around a line that luluwrites uses in her fic "Bones, Picked Clean." I have to give credit because she's inspired me so much as a writer (and also it's her line). If you haven't already, go and read everything she's written. It's all great.

There’s a pull in his soul. The edges of his vision turn to a hazy black. He can see the carpet beneath his feet, but he knows it won’t be there for long. There’s no shock. No denial. No bargaining. He should be _angry_. Stars, he _wants_ to be angry. But he can’t be. Not when he knew this was coming. He wanted to believe he was paranoid, that the kid meant their promise. This is the farthest they’ve ever gotten. He still remembers the butterflies, the _hope_ he had when he realized they hadn’t made it past that date before. It was too good to last. He _knew_ that. So when this timeline fades away into nothing, he feels nothing.

 

Then it stops.

 

He’s still in his room. _His_ room, not his old one. His room with the light blue walls covered in posters. The one where his bed has a frame, his desk has a computer, and he wakes up with sunlight streaming through his window. Relief floods his bones.

It… stopped.

He fumbles with his phone in his pocket. No time was lost. Not even a minute. 

Did it not work? 

He calls on his magic, shortcutting downstairs. He doesn’t know what he’s looking for, but his eye lights scan the room.

_*something’s different. what’s different?_

Pap isn’t here. It’s nothing to get worked up about, though. He’s usually at work now, anyway.

No, there’s something eating at the back of his soul. It’s like the stairs have been lowered by a centimeter or the furniture has been moved half an inch to the right. There’s something he usually feels, something he _expects_ to feel, that isn’t there. 

He feels his phone buzz and digs it out of his pocket.

Frisky Business: Did you feel that?

_Bzzt_.

Frisky Business: What happened?

Sans feels his magic vibrating in his bones as he types out his response.

(xxx)xxx-xxxx: stay put.

His slippers hit the tile floor of the lab with a soft pat. “what’s up, doc?”

“Sans. What a pleasant surprise.” It doesn’t sound very pleasant. The doctor doesn’t even look up from his notebook when he greets him. He adjusts his glasses as his long, spindly fingers scan the page. “I was wondering when you’d grace me with your presence.”

“how’s work?”

“Working.” He scribbles something in the margin and makes a thoughtful sound.

He didn’t lie. He knew Sans could tell if he was. Gaster preferred to circumvent the truth, answering all questions completely honestly, but without any of the desired information. Sans forgot how much he hated that.

“find anything worth sharing?” He shoves his hands into his pockets, keeping his lazy grin in place. 

“You sound like you think I did.” 

“i know you did.”

The doctor exhales an exasperated sigh, massaging his temple with a gloved hand. “Sans, if there’s something you wish to ask me, just do it. I don’t have time to play games.” 

Stars, he _really_ forgot how much he hated that.

He takes a moment to look over the lab. It’s messier than the last time he was here, papers strewn across every available surface. His eyes stop at a desk in the corner. There’s a fresh cup of coffee on it, steam still coming off the top.

“say doc, where’s your assistant?”

“If I had to guess, I’d say an alternate reality. I doubt she’ll be coming back anytime soon if that’s what you were wondering.”

Sans feels the magic drain from his bones. Gaster didn’t even bother to look up from his book. His hands curl into fists inside of his pockets.

There’s a soft ding.

_That_ gets his attention. He finally drops his pencil, turning to look the short skeleton directly in the sockets. “Blue magic? Really, Sans? I know this body makes me look young, but I’m _hardly_ in stripes anymore.” 

Stars, he fucking hated that _drawl_. He clenches his fists tighter.

“you mind telling me a little more about this ‘alternate reality’?” He’s not going to lose his temper. Not now. Not for _him_. Still, he finds himself pulling his grin a little tighter at the edges.

Gaster chuckles, his laugh bouncing off the concrete walls. “Where are your manners, Sans?” 

“aw, shucks you know what, doc? i think i must’ve left ‘em at home. guess we’ll have to do without.” 

He clicks his tongue. “Pity. I do miss when we could be civil.” 

His hands threaten to turn to dust in his pockets. “can’t say i remember.”

For a moment, a spark of purple lights up in the doctor’s usually dark eyes. He straightens his glasses and the light disappears. 

“My assistant and I had managed to create a SAVE a while back. She’s not Determined, but her intuition was such that she was able to create one anyway (with some help of course).” The smile he flashes is sickening, but he continues. “The experiment was ultimately a failure. Interesting, but a failure nonetheless. It’s a shame, she was _such_ a helpful assistant… _lovely_ girl, too. But I’m sure you knew _all_ about that.”

Sans feels his soul stop in his chest. You were just supposed to translate notes and file papers. You weren’t supposed to-

“And since I _know_ you’ll ask, I might as well answer anyway and save us _both_ some hostility. The void swallowed her in transit. I’m sure it spit her back out somewhere nice.” He turns back to his notes, seemingly unaffected.

_*he doesn’t know where you are. he didn’t even look._

The next words come out strained through a clenched jaw. “and if it didn’t?”

Gaster flashes a broad grin, a grin Sans knows far too well.

“Well, they can’t all be winners now, can they?”

**Notes for the Chapter:**

> I didn't originally plan for this chapter to happen so early on in the story, but uh screw suspense I guess? I don't know and I don't care! This is really helping me get through my writer's block so I'm going to ride the inspiration train as long for as long as it takes me! And yes, I will update my other fics (and finish them). It's just a slow process. 
> 
> Hope you enjoy!


End file.
